1. Field of the Invention
The invention is in relation to electrostatic image formation apparatus and in particular, refers to the adjustment of electrifier which charges the surface of the photo conductor.
2. Description of the Prior Art
Electrostatic image formation apparatus as a background of the invention, charges the photo conductor by the applied voltage on electrifier installed along the rotating photo conductor, and this apparatus is of the type that produces an electrostatic latent image by projection of light on the charge. Such electrostatic latent image is developed by the toner when passing through the developing section. In order to apply the toner to the electrostatic latent image, it is the condition that the electric potential of the charge given to the photo conductor is adjusted at the fixed rate. For example, a certain copying machine is required a potential difference of more or less 800 V.
The charge on the surface of such photo conductor is usually supplied by corona discharge brought about by the applied voltage of the electrifier. Under the circumstances, an apparatus to project the light have to be installed between the developing devices and said electrifier. Therefore, the electrifier is placed a little away from the developing device.
In recent years, however, photo conductors made of amorphous silicon are used very frequently by reason of their non-pollution and high durability. However, photo conductors made of amorphous silicon have a fault that are inferior in the characteristics of the potential falling to the other photo conductors.
Therefore, in copying machines and like for which electrifiers are placed a little away from the developing section, electric potential falls by tens of volt from the point when charged by corona discharge till the arrival at the developing section, and as the results, photo conductors of amorphous silicon had a fault that a desired electric potential cannot be secured for the most suitable photographic image density at the developing section.
Therefore, for example, in the electronic photograph image formation apparatus which appears in the Government Bulletin No. Japanese Laid-Open Patent Publication (unexamined) No. 61-281258, photo conductors are charged for a fixed amount before the actuation for the formation of image and at the time the part charged arrived at the position of the surface electric potential sensor, said apparatus stops the rotation of the photo conductor for a fixed time and detect the potential falling characteristics of the photo conductor.
As stated above, the characteristics of potential falling can be measured, but, in fact, the relation (charge characteristics) between voltage charged on electrifier and charge amount generated by photo conductor as the result is changed, charge amount desired at the developing section can not be obtained.
Such relations between voltage charged on electrifier and charge amount generated on photo conductor, depend on ambient atmospheric temperature, deterioration with the lapse of time of photo conductor and the like and besides, the fact is well known that there are a great deal of dispersion in said relation of the individual photo conductors.
In the corona current setting system appearing on the Government Bulletin No. Japanese Laid-Open Patent Publication (unexamined) No. 60-55363, taking note of the dispersion owing to change of the ambient atmospheric temperature as given above, a data table is prepared calling for the relations between the ambient atmospheric temperature and charge amount caused according to the voltage to be fixed and the voltage of electrifier is adjusted in accordance with the change of ambient atmospheric temperature on the strength of the above information of data table.
The current setting system appearing on the above Government Bulletin No. Japanese Laid-Open Patent Publication (unexamined), has superiority in that the adequate electrifier voltage (charge characteristics) can be set in accordance with the change of ambient atmospheric temperature. On the other hand, however, as stated already, the relation between electrifier voltage and charge amount generated on the surface of photo conductor drastically change not only by the change of the ambient atmospheric temperature, but also by the dispersion of the characteristics of the individual photo conductors and its aged deteriorations. In the circumstances, a means for copying with all the changes, as stated on the Government Bulletin No. Japanese Laid-Open Patent Publication (unexamined) No. 60-55363, the method to prepare data table in advance requires a huge amount of memory capacity and at the same time, complicates the programs for operation and logical judgement and is not economical.
Accordingly, the first object of this invention is to keep up with the change of photo conductor characteristics without preparing data table for the cause of change.
As above, the amount of potential falling changes exorbitantly with the lapse of time, and the error becomes a large amount without the use of the latest measured value. On top of that, the amount of potential falling will rely to a great extent, on the temperature of the photo conductor. Therefore, it is advisable to revise the amount of potential falling in conformity with the change of temperature of photo conductor at the time the continuous copying process is performed.
In the advanced technology disclosed in the Government Bulletin No. Japanese Laid-Open Patent Publication (unexamined) No. 60-55363, however, the charge performance data table is only the item prepared and potential falling characteristics is not thus resulting in the inability to the accurate revision of potential falling and besides, it was not able to renew the data table to catch up with change in the lapse of time.
For that reason, the second object of this invention is to offer the image formation apparatus that have the following features; the amount of revision can be changed for the potential falling in order to follow the aged deteriorations of the photo conductor and at the same time, an adequate revision for the potential falling to keep up with the temperature change of photo conductor in the course of continuous copy processing.
The FIG. 11 shows an example with a graph of the condition of the fall of electric potential (potential falling) for .DELTA.t seconds of photo conductor surface. Generally as shown in FIG. 11, the manuscripts in "Solid black" and in "Intermediate tone" are different in their amount of electric potential falling for the photo conductor's fall of potential and we can see a change in the amount of electric potential falling for the fall of potential caused by the change of the temperature in photo conductor.
In order to bring the value of electric potential of the photo conductor surface to the desired value, several methods have been studied to control an applied level of electric potential of the aforesaid electrifier higher by the amount of potential falling of photo conductor and in this case, said level should be controlled usually the above electrifier applied voltage to get the desired image density of "Solid black."
The FIG. 9 shows the electric potential of photo conductor at the developing section when the above-mentioned conventional electrostatic image formation apparatus is used.
As per the Fig., the electric potential of photo conductor for "Solid black" at the developing section is now controlled to be objective electric potential (e.g., 800 V).
However, the electric potential of the photo conductor for "Intermediate tone" at the developing section will be away to a great extent from the objective electric potential (e.g., 250 V) with a raise of temperature of photo conductor.
Therefore, the picture appearance quality will be drastically deteriorated at the "Intermediate tone" on account of the unavailability of the desired image density.
The FIG. 10 shows the electric potential at the developing section when controlled to secure the desired image density at the "Intermediate tone."
In this case, the electric potential of photo conductor is controlled at 250 V of the objective electric potential at the developing section. However, as shown in the drawing, the electric potential of photo conductor for "Solid black" will be away to a great extent from 800 V of objective electric potential with a raise of temperature of photo conductor.
Accordingly, the picture appearance quality will be drastically deteriorated by the non-availability of the desired image density in the "Solid black."
Consequently, this invention's third object is to offer the electrostatic image formation apparatus that can get the desired image density, in both the "Solid black" and "Intermediate tone" even in the change of photo conductor temperature.
As previously stated, it is a fact well-known that the amount of potential falling of photo conductor and charge characteristics are changeable with several causes. On the contrary, as mentioned before, the amount of potential falling rely heavily on the temperature of photo conductor and accordingly changes to a great extent, and charge characteristics is also changeable on a vast scale according to the degree of deterioration (Deterioration with a lapse of time) of photo conductor.
Therefore, the fourth object of this invention is the charge characteristics of photo conductor which is to be measured timely at the time power source is set on, and at the finish of image processing and another is the amount of potential falling, an accurate voltage value of which is to be figured out by referring to storage cell in proportion to the temperature of photo conductor.